Prolonged ureteral occlusion may lead to progressive loss of renal parenchyma and function. The degree of recovery of renal function after relief of obstruction is inversely related to the duration of obstruction. Besides a progressive decrease in GFR and renal blood flow, the effects of ureteral occlusion include, a failure to maximally conserve water and sodium and to excrete acid. Marked alterations in the reabsorption of phosphorus, calcium and magnesium also occur following release of acute bilateral and unilateral ureteral obstruction. The proposed studies will utilize the Munich-Wistar rat which has an accessible renal papilla and the technique of micropuncture to assess the effects of ureteral obstruction on surface nephrons, deep nephrons, and the terminal segment of the collecting duct. The particular areas to be studied include: 1) the renal acidifying defect observed with obstructive uropathy; 2) to analyze the renal tubule segmental reabsorption of calcium and magnesium by surface and deep nephrons after release of unilateral and bilateral ureteral obstruction; 3) to determine the renal tubule sites where changes in phosphate reabsorption occur after the onset of unilateral or bilateral ureteral obstruction and the mechanism underlying the hypophosphaturic responses to parathyroid hormone after release of unilateral obstruction. Through these studies it is hoped to develop a clearer understanding of how ureteral obstruction alters the capacity of the kidney to handle different anions and cations. These observations, in turn, might suggest new therapeutic modalities and improve approaches to patient management.